BEHAVIOR OF UNICELLULAR ORGANISMS. 105 



upon this point showed that the chief factor determining 

 whether a substance does or does not cause the motor reflex 

 of Paramecium is not the injuriousness of the substance, but is 

 of a chemical nature. 



We are now prepared to sum up the main results on Para- 

 mecium. In this animal we find that chemotaxis, thermotaxis, 

 tonotaxis, reactions to mechanical shock, and the like, are not 

 distinct kinds of activity ; that in each case we have the same 

 movements, merely induced by different agents. When Para- 

 mecium is effectively stimulated by any substances acting 

 chemically or through osmosis, by heat or by cold or by 

 mechanical shock, it responds with a reflex, which consists of 

 the following activities : the animal swims backward, turns 

 toward its own aboral side, then swims forward. The result of 

 this method of reaction is that the Paramecia tend to leave 

 the sphere of influence of agents causing this reflex, and to 

 congregate in areas where this reaction is not caused. For 

 chemical substances at least it is proved that the position of 

 the stimulating agent has no influence on the direction of move- 

 ment after a stimulus ; the direction of movement throughout 

 the reaction is determined by internal factors. 



Is this reaction method one that is common among unicellu- 

 lar organisms, or is it peculiar to Paramecium ? To answer 

 this question I have studied the reactions of a considerable 

 number of unicellular organisms belonging to the Flagellata 

 and Ciliata. The essential point in the reaction of Paramecium, 

 the factor that gives character to the entire response, is the 

 circumstance that the animal after stimulation turns toward one 

 side which is structurally defined, without regard to the nature 

 and position of the source of stimulus. The point to which 

 attention was primarily directed in studying the other organ- 

 isms was, therefore, whether after stimulation the creature 

 turned always toward one structurally defined side. 



The organisms studied included, among the flagellates : Chi- 

 lomonas paramecium and Englena viridis ; in the ciliates the 

 following Holotricha : Paramecium caudatum,Loxophyllum mele- 

 agris, Colpidium colpoda, Microthorax sulcatus, Dileptus anser, 

 Loxodes rostrum, and a species of Prorodon ; the following Hete- 



